Abstract
Stoichiometric barium strontium titanate (Ba 0.5Sr 0.5TiO 3 or BST) 220-nm thick thin films were deposited by pulsed laser ablation on SiO 2/Si, RuO 2/Ta/SiO 2/Si and Pt/Ti/SiO 2/Si substrates at 400°C. The films were weakly crystalline as-deposited. Crystallization was induced by annealing the films in the range of 550–650°C. The BST films deposited on Pt/Ti/SiO 2/c-Si substrates presented wide cracks that were promoted during the annealing process due to the thermal expansion mismatch between the BST films (α BST=4×10 −6 °C −1) and the Pt (α Pt=9×10 −6 °C −1). Smooth films showing slightly cracked areas were obtained on SiO 2/c-Si and RuO 2/Ta/SiO 2/Si substrates. The ruthenium oxide thermal expansion coefficient is α RuO 2 =5.2×10 −6 °C −1. A cross-sectional analysis at the ferroelectric/substrate interface showed that for the lower annealing temperature (550°C) a mixed amorphous/nanocrystalline microstructure is formed. For temperatures above 600°C a randomly oriented polycrystalline material is obtained. However, an amorphous layer of 4–6 nm still remains on the substrate even after heat-treatments up to 650°C. The dielectric constant of the BST films varied in the range of 30–325.
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